Six experiments are proposed to investigate the psychological processes underlying the perception of speech in humans. These experiments will attempt to clarify present notions of the levels of organization in the speech perception system by employing the technique of selective adaption and a test series of stimuli which vary from speech-to-nonspeech. These stimuli can be described in terms of simple auditory patterns, complex auditory properties, or in perceptually meaningful terms; each set of terms corresponds to a set of feature detectors which have been proposed to mediate each stage of perceptual processing. Selective adapatation tests will be performed to assess the relative contributions of detectors at various levels to the perceptual changes occasioned by fatigue. The general suitability of the levels-of-processing conception of the perceptual process will also be considered in the light of the changes in the perception of the test continuum, which bridges perceptual modes, from speech to general auditory perception. The outcome of this work will provide a strong test of the theory of feature detectors in speech perception, and will clarify current conceptualizations of the interchange between speech perception and nonspeech auditory perception.